Isolators are devices that allow microwave energy to pass in one direction with little loss but absorb power in the reverse direction. They minimize reflections between cascaded components with different impedances. Isolators are generally made from circulators with one port connected to a 50 ohm impedance. Such isolators are made of ferrite passive devices. In monolithic circuit applications, they are made of hybrid integrated circuits and tend to be very bulky in that they are one or two orders of magnitude larger than the associated cascaded circuits.
In microwave applications, it is desirable to have an isolator which can be fabricated on a monolithic chip, particularly GaAs, along with the circuit components to be cascaded. Heretofore, this has not been possible with conventional isolators.